reverse: add single-entry path cache

Speeds up the "ls -lR" benchmark from 2.6 seconds to 2.0.

internal/fusefrontend_reverse/rpath.go

@@ -82,18 +82,31 @@ func (rfs *ReverseFS) decryptPath(relPath string) (string, error) {
 	if rfs.args.PlaintextNames || relPath == "" {
 		return relPath, nil
 	}
+	// Check if the parent dir is in the cache
+	cDir := saneDir(relPath)
+	dirIV, pDir := rPathCache.lookup(cDir)
+	if dirIV != nil {
+		cName := filepath.Base(relPath)
+		pName, err := rfs.rDecryptName(cName, dirIV, pDir)
+		if err != nil {
+			return "", err
+		}
+		return filepath.Join(pDir, pName), nil
+	}
 	parts := strings.Split(relPath, "/")
 	var transformedParts []string
 	for i := range parts {
 		// Start at the top and recurse
 		currentCipherDir := filepath.Join(parts[:i]...)
 		currentPlainDir := filepath.Join(transformedParts[:i]...)
-		dirIV := derivePathIV(currentCipherDir, ivPurposeDirIV)
+		dirIV = derivePathIV(currentCipherDir, ivPurposeDirIV)
 		transformedPart, err := rfs.rDecryptName(parts[i], dirIV, currentPlainDir)
 		if err != nil {
 			return "", err
 		}
 		transformedParts = append(transformedParts, transformedPart)
 	}
-	return filepath.Join(transformedParts...), nil
+	pRelPath := filepath.Join(transformedParts...)
+	rPathCache.store(cDir, dirIV, saneDir(pRelPath))
+	return pRelPath, nil
 }

internal/fusefrontend_reverse/rpath_cache.go

@@ -0,0 +1,41 @@
+package fusefrontend_reverse
+
+import (
+	"sync"
+)
+
+// rPathCacheContainer is a simple one entry path cache. Because the dirIV
+// is generated deterministically from the directory path, there is no need
+// to ever invalidate entries.
+type rPathCacheContainer struct {
+	sync.Mutex
+	// Relative ciphertext path to the directory
+	cPath string
+	// Relative plaintext path
+	pPath string
+	// Directory IV of the directory
+	dirIV []byte
+}
+
+func (c *rPathCacheContainer) lookup(cPath string) ([]byte, string) {
+	c.Lock()
+	defer c.Unlock()
+	if cPath == c.cPath {
+		//fmt.Printf("HIT   %q\n", cPath)
+		return c.dirIV, c.pPath
+	}
+	//fmt.Printf("MISS  %q\n", cPath)
+	return nil, ""
+}
+
+// store - write entry for "cPath" into the cache
+func (c *rPathCacheContainer) store(cPath string, dirIV []byte, pPath string) {
+	//fmt.Printf("STORE %q\n", cPath)
+	c.Lock()
+	defer c.Unlock()
+	c.cPath = cPath
+	c.dirIV = dirIV
+	c.pPath = pPath
+}
+
+var rPathCache rPathCacheContainer